Infection, malignancy and inflammatory disorders are often accompanied by elevated plasma concentrations of interleukin-6 (IL-6), and thrombotic disease is not uncommonly associated with such conditions. A variety of experiments has shown that administration of IL-6, a pro-inflammatory cytokine with a broad range of biological effects, alters multiple aspects of the hemostatic mechanism. These changes include augmentation of both the platelet count and the sensitivity of platelets to activation by strong agonists, increases in plasma fibrinogen and von Willebrand factor concentrations, and decreases in free protein S. In toto, these alterations in both the platelet and coagulant phases of hemostasis can be interpreted as pro-hemostatic, and perhaps pro-thrombotic. Moreover, several cytokines in the IL-6 family have similar effects. Based on these observations, the major objectives of this proposal are to investigate the hypotheses that: 1) elevated IL-6 levels enhance thrombogenesis in vivo; and 2) blockade of the action of IL-6-like cytokines by antibody- induced inhibition of IL-6-mediated signal transduction will reverse their physiologic and potentially pathological consequences. The hypotheses will be evaluated by using an in vivo canine model whereby short-term administration of IL-6 is used to approximate disease-related endogenous elevations of the IL-6 family of cytokines. Four specific aims will be pursued. In Aim 1, IL-6 will be given to dogs with a thrombotic device inserted into an arterio-venous shunt. Deposition of components of thrombi, including platelets, fibrinogen, red cells and white cells will be measured. The effects of IL-6 will be compared to those of the thrombopoietic factor mpl ligand. In Aim 2, attempts to reverse the pro- hemostatic effects of IL-6 will be made by administration of neutralizing monoclonal antibodies to gp130, the common signal-transducing chain of the IL-6 family of receptors. Inhibition of the pro-hemostatic effects will be monitored by analysis of platelet activation and ELISAs for plasma hemostatic components. In Aim 3, these antibodies will be used in the dog thrombosis model to determine if the putative pro-thrombotic effects of IL-6 can be inhibited. In Aim 4, the minimal epitope that the antibodies recognize will be mapped. Successful pursuit of these aims may permit increased understanding of the relationship between the IL-6 family of cytokines and thrombogenesis, and provide novel therapeutic approaches to thrombotic disease.